To for instance state that RuneQuest would clearly help them get a better simulation without even knowing what they try to simulate seem like a quite extraordinary claim.
Ahh, this nails down the issue here.
I am in no way making any sort of qualitative judgement about better or worse. I'm not. I'm in no way saying that RuneQuest (for example) would be a "better" system to use for simulation.
No.
I'm saying that mechanics which do not provide any input into how the result was achieved are not simulationist mechanics. Which, really, covers the idea that the DM provides the input. After all, a freeform system where the DM simply provides the narrative is, in a way, a simulation.
But, that doesn't change the fact that the simulation is completely divorced from the mechanics that created that result. The mechanics are simply giving you some sort of result. Which means that the mechanics are not simulating anything. And we know they aren't simulating anything because if they were simulating something, we would have some indication as to how that result occured.
You keep trying to take this to a higher altitude and say, "Well, because the DM is providing the narrative, then the system is simulationist because the system includes the DM". Which, fair enough I suppose. That's a pretty solid argument. But, my arguement is that while the DM might provide the narrative, that doesn't mean that the mechanics are now suddenly simulationist. After all, in pretty much all RPG's, the DM/GM will provide the narrative. But, in some systems, that narrative is generated in part by the system itself, rather than just free form added on after the fact.
And it is those systems, which generate some part of the narrative, which are what I consider simulationist.
@AlViking has repeatedly pointed to the blog post that dovetails nicely with what I'm saying. Simulationist mechanics must be diegetic. They must provide some information as to how something happened. That's what diegetic means. Something cannot be diegetic without providing some clue as to how a result was achieved.